Thousands of people living around the Japanese nuclear power plant most seriously damaged by the earthquake evacuated Saturday as authorities detected radiation leaks from the crippled facility, began screening evacuees for radiation exposure and announced plans to distribute pills to protect against thyroid cancer.

Government officials stressed that the amount of radiation that had been released by the Fukushima Daiichi Unit 1 reactor after an explosion at the facility appeared to be relatively low, and international authorities said the situation had not yet become a major public health threat.

To try to assess the extent of the exposures, workers at evacuation centers - wearing white masks and protective clothing - used handheld scanners to check everyone for radiation exposure as an estimated 170,000 people fled a evacuation zone that had been doubled to a 12-mile radius around the plant. The number of people possibly exposed to radiation could reach 160, the Japanese nuclear safety agency said.

"They must be planning for a worst-case scenario, which would be the core partially melting down or melting down," said John Boice Jr., scientific director of the International Epidemiology Institute in Rockville.

Three of those known to have been exposed had been chosen for random testing from among 90 patients and staff members at a hospital two miles from the plant who were awaiting evacuation by helicopter and needed to be decontaminated, officials said. None had yet shown physical symptoms of radiation poisoning, officials said.

As authorities in Japan know better than those in any other nation, the extent of the risk from a nuclear power plant ultimately depends on how much and what kind of radioactive material is released, where it travels and how many people are exposed for how long, experts said.

"Anything having to do with health effects has to do with the amount of exposure the population receives, and that's just an unknown," Boice said. "That's determined by many factors, including which way the wind is blowing."

Radiation from nuclear power plants poses a host of health risks, ranging from severe toxic effects in workers exposed to high doses to long-term increased rates of many cancers, experts said.

Most of what is known about the risks of radiation comes from studying survivors of the atomic bombings of Hiroshima and Nagasaki during World War II, the 1986 Chernobyl nuclear disaster in Ukraine and radiation exposures for medical purposes.

In Chernobyl, about 30 firefighters who were exposed to very high doses of radiation while trying to douse the blaze at the plant died within a month.

"They had huge exposures," Boice said. "These were the kind of doses that just knocked out the blood system, the gastrointestinal system."

One worker at the Daiichi plant had died from injuries after becoming trapped in the exhaust stack of the plant, according to the World Nuclear Association. At least four other workers were reportedly injured in the explosion and had been hospitalized.

Beyond the deaths of firefighters in Chernobyl, the most well-documented health effect was an increase in thyroid cancer, primarily among children, due to exposure to iodine-131. An estimated 6,000 to 7,000 excess cases of thyroid cancer have occurred because of Chernobyl, mostly among people who were children at the time.

"At Chernobyl, the biggest problem was it got on the grass and the cows ate it and the milk from the cows was given to the kids," said Fred Mettler, a radiation expert at the University of New Mexico.

Japanese officials announced plans to distribute potassium iodide pills, which block radioactive iodine from accumulating in the thyroid glands, causing thyroid cancer, to people living around the Fukushima Daiichi facility and another damaged plant about seven miles away.

Of the radioactive elements released in a nuclear plant leak, radioactive iodine has a relatively short "half-life" of eight days, which means that it essentially disappears within about 80 days, Mettler said. In comparison, another radioactive substance released by nuclear power plants, cesium-137, has a half-life of about 30 years, meaning it poses a much greater risk because it gets into the food chain.

In the Hiroshima and Nagasaki atomic bombings, which provide some of the best data about the risks of radiation, about 80,000 people died from the blasts and exposure to very high doses of radiation. Studies of the survivors found that rates of a variety of cancers, including leukemia and cancer of the breast, lung and colon, remained elevated for decades.

"The Japanese have been studying radiation since they dropped the bomb over there," Mettler said. "They have been following the atomic bomb survivors for 60 years. They are the world experts on radiation effects. So nothing is lost on them."

Excess cases of leukemia begin to show up within two years of exposure and peak within five to 10 years; other cancers do not start to appear in excess for at least a decade, and their risk can remain elevated for decades. But even in Hiroshima and Nagasaki, only about 9,000 survivors have died from cancer. That's about 500 more cases than would have been normally expected, Mettler said.

Studies have also found adverse psychological effects from nuclear accidents, such as the Chernobyl fire and the 1979 Three Mile Island meltdown in Pennsylvania. For example, there was an increase in suicides in Estonia and among cleanup workers after Chernobyl. Abortions also increased in some places.

"It turns out there wasn't increases in birth defects or malformations in the surrounding populations, but there was an increase in elective abortions because people were so concerned," Boice said.